The present invention relates to a system for determining a measure of the physical activity of an object over a certain period of time, said system comprising measurement means for measuring data relating to movement of the object, data processing means for processing the measured data and memory means for storing the processed data. The present invention also relates to a method for determining a measure of the physical activity of an object over a certain period of time comprising the steps of:
a) measuring data relating to movement of the object;
b) processing the measured data, and
c) storing the processed data.
The physical activity of an object, notably a human being, is an important determinant of its health. The amount of daily physical activity is considered to be a central factor in the etiology, prevention and treatment of various diseases. Information about personal physical activity can assist the individual in maintaining or improving his or her functional health status and quality of life.
A system and a method according to the preamble are described in the article xe2x80x9cA Triaxial Accelerometer and Portable Data Processing Unit for the Assessment of Daily Physical Activityxe2x80x9d, by Bouten et al., IEEE Transactions on Biomedical Engineering, Vol. 44, NO. 3, March 1997.
According to the known system and method a triaxial accelerometer composed of three orthogonally mounted uniaxial piezoresistive accelerometers is used to measure accelerations covering the amplitude and frequency ranges of human body acceleration. Thereto an individual wears the triaxial accelerometer over a certain period of time. A data processing unit is attached to the triaxial accelerometer and programmed to determine the time integrals of the moduli of accelerometer output from the three orthogonal measurement directions. These time integrals are summed up and the output is stored in a memory that can be read out by a computer. The output of the triaxial accelerometer bears some relation to energy expenditure due to physical activity and provides as such a measure for the latter.
The known system allows for a complete and accurate measurement of human body acceleration in three directions. Using state of the art techniques in the field of integrated circuit technology the accelerometer can be build small and light weight allowing it to be worn for several days or even longer without imposing a burden to the individual wearing it.
However the known system and method have the disadvantage that the output thereof is in arbitrary units. The outputs of several accelerometers worn by different individuals are thus not comparable in the sense that no quantitative conclusions can be drawn thereon. In fact the same is valid for the outputs of one accelerometer worn by a single individual during different time periods.
It is an object of the invention to provide a system and a method according to the preamble that provides a reproducible and objective measure of the physical activity of an object over a certain period of time.
The system according to the invention is thereto characterized in that it further comprises calculating means for calculating the physical activity level of the object. The method according to the invention is thereto characterized in that it further comprises the step of: d) calculating the physical activity level of the object.
It is known in the relative field that physical activity can be expressed as PAL (Physical Activity Level). PAL is defined as Average Daily Metabolic Rate (ADMR) divided by Basal Metabolic Rate (BMR). The ADMR (or total daily energy expenditure) can be divided into three compartments: BMR, DEE (Diet Induced Energy Expenditure) and AEE (Activity Induced Energy Expenditure). BMR is the amount of energy spent in rest (not asleep) and is strongly correlated with body weight. DEE is the thermogenic effect that occurs after food ingestion and is, for a subject in energy balance, approximately 10% of ADMR. AEE is the amount of energy spent on physical activity and is the most variable of all components and the most difficult to measure. Thus, PAL is a relative and objective number reflecting an individuals activity and is therefore suitable for use in comparison of physical activity between different individuals.
Currently measurements of ADMR are performed by using the known doubly labeled water method. Doubly labeled water (2H218O) however is difficult to obtain and expensive. ADMR can also be determined by calculation of heat production from O2 consumption and CO2 production for which purpose an individual has to be confined to a respiration chamber for a period of up to several days. BMR can be determined by calculation of heat production from O2 consumption and CO2 production measured in a respiration chamber or as measured with a ventilated hood. The BMR can also be calculated with known formulas using age, sex, height and weight. Clearly measurement of ADMR and thus PAL is impractical in daily life and certainly not feasible for population studies.
Contrary thereto the invention provides a system and a method to calculate the physical activity level of an individual in a practical manner that can be used in daily life by everyone who is interested in optimizing his health. The system and method according to the invention can for example be used at home for diet adjustment, by a medical professional to improve diagnosis or adjust medication levels in case of diseases, such as coronary diseases or Parkinson""s disease, or sleep disorders. The use is certainly not limited to human beings but can be extended to non-living objects, such as buildings or machines, for instance to measure vibrations.
In a first preferred embodiment of the system and the method according to the invention the calculation of the physical activity level of the object is based on the following algorithm:
PAL=factor 1+factor 2*(variable 1)factor 3+factor 4*(variable 2)factor 5+ . . . +factor (2n)*(variable n)factor(2n+1)
wherein
PAL=physical activity level of the object;
Variable 1 through variable n=variables related to the object and/or the system; and
Factor 1 through factor 2n+1=factors to be determined by validation of the system.
According to the first embodiment advantageously all relevant variables relating to the individual object and the system can be accounted for to obtain the physical activity level as an objective measure.
A preferred embodiment is directed to a living subject and the variables related to the subject comprise body mass and/or sleep time and/or age and/or length and/or sex. Preferably the variables related to the system comprise output and/or wear time of the measurement means. The output preferably firstly is processed by the data processing means before being input to the memory means or directly to the calculating means.
From experiments it has been shown that the following algorithm reveals an accurate number for the physical activity level of a living subject:
xe2x80x83PAL=a+b*(output)c+d*(body mass)e+f*(sleep time)g+h*(wear time)I
An even more accurate assessment of the physical activity level of the subject can be obtained with the following algorithm:
PAL=a+b*(output)c+d*(body mass)e+f*(sleep time)g+h*(wear time)I+j*(age)k+l*(length)m+n*(sex)o
According to a preferred embodiment of the system according to the invention the measurement means comprise three orthogonally mounted accelerometers for measuring the acceleration of the object. Measuring the acceleration in vertical, antero-posterior and medio-lateral direction allows for a complete registration of the acceleration of the object.
In another preferred embodiment the accelerometers comprise piezo-electric material allowing the measurement of acceleration without the need for an additional power supply. Preferably the piezo-electric material is uni-axial in order to truly confine the measurement of each accelerometer to one direction. More preferably the piezo-electric material is serial dimorph thus enhancing the sensitivity of the measurements.
In an elegant preferred embodiment the accelerometers have a strip form and are fixed at one end thereof. The mutually orthogonal positioning of the accelerometers can now be realized in a practical way. Furthermore due to this fixation the sensitivity of the accelerometers is relatively high and can be easily enlarged by placing weights at the outer ends of the strips.
In a practical embodiment the system according to the invention comprises an activity monitor in which the measurement means, data processing means and memory means are incorporated, which activity monitor further comprises communication means for communicating with the calculating means. Preferably the calculating means comprise a computer.
The invention also relates to an activity monitor described as part of the above-described system.
The invention also concerns a computer program to carry out one or more of the steps of the method according to the invention.